This invention relates to a method of controlling the supply of power to electrical loads with a minimum of switching surges, more particularly heating loads which are each electrically subdivided into at least two equal subloads, in which the subloads of each load can be connected arbitrarily in at least three main power stages in series arrangement alternately individually or in parallel arrangement to an A.C. mains.
For the control of power supplied to electrical loads having a high power consumption, for example furnace plates of an electric furnace, it is frequently desirable to make available a maximum number of finely graded power stages. For this purpose, it is known, for example, for furnace plates to provide a so-called three-point control, in which several windings are arranged in parallel, which admit different power consumptions. Such an arrangement is very complicated, however, and cannot be used economically, especially for many power stages.
EP No. A2 0 162 620 discloses the method mentioned in the opening paragraph, in which each load is subdivided into several subloads, that is to say, for example, that a heating winding is subdivided into several subwindings in such a manner that the subwindings can be connected to the supply voltage mains in series arrangement, in series-parallel or in parallel arrangement. If then each heating load is subdivided into two subloads, a main power stage is obtained for each heating load. If the subloads are connected to the mains in parallel arrangement, they have the maximum current consumption. The power stage having the smallest power consumption is the series arrangement, in which both windings are connected in the mains in series arrangement. In such a method of controlling the power supply, several main power stages are thus possible with heating loads subdivided into two subloads. Such a power gradation is frequently not sufficient, however, for some applications, for example for furnace plates of an electric furnace.
For the control of power with finer gradation of the power consumption, it would be possible to switch to and fro between the main power stages or to switch on the main power stages each time only for given periods. However, due to the high power consumption, the problem of mains polution arises. When switching the loads on and off, in fact switching surges are produced which give rise to corresponding mains voltage fluctuations. The eye reacts very sensitively to such mains voltage fluctuations, especially in a frequency range below 25 Hz. Therefore, for corresponding apparatuses there are regulations which limit the value of the switching surges and thus the mains pollutions caused thereby. A relevant regulation is the IEC standard 555 "Disturbances in supply system caused by household appliances and similar electrical equipment". This standard defines the content of higher waves frequency (above 50 Hz) and mains voltage fluctuations below 50 Hz which may be allowed by electrical loads connected to a mains. Moreover, a very small effect of the net direct current is prescribed.
In practice, the problem arises that for the maintenance of such standards especially for loads above 500 W, shorter switching times between the aforementioned main power stages are, practically speaking, no longer possible because then the standard is no longer maintained. It should be noted that the mains voltage fluctuations caused be devices having a high power consumption are frequently designated as "flicker".
With the power stages described in the aforementioned EP-No. A2 0 162 620, the IEC standard cannot be maintained because the effects of switching surges are too large when switching between different stages and moreover an inadmissible net direct current effect occurs.
DE-No. OS 3426046 further discloses an electrical continuous heater in which two power stages are provided. The lower power stage is constituted by a load consisting of two parallel-connected heating resistors which are connected to one phase of a three-phase current mains. The higher power stage is constituted by heating resistors which are arranged in a star circuit and are connected to the phases of the three-phase current mains. Therefore, two main power stages are realized by more than two subloads. The higher power stage can be realized not by an A.C. mains, but only by a three-phase current mains.